This invention relates to whole surface driven speakers in which a diaphragm carrying a strip-shaped coil is disposed between two groups of permanent magnets having opposing magnetic pole surfaces.
The whole surface driven speakers of the kind referred to can be effectively employed in acoustic equipment of a relatively wide frequency band.
Various types of whole surface driven speakers have been suggested, an example of which is the one disclosed in U.S. Pat. No. 3,922,504 to Kenichiro Kishikawa et al. According to this U.S. patent, an electro-acoustic transducer comprises a diaphragm formed by providing a series of strip shaped conductor coils on a sheet of insulating material. Two groups of elongated permanent magnets are arranged on both sides of the diaphragm so as to oppose each other. Pole faces of the same polarity pole oppose each other across the diaphragm but pole faces of opposite polarity are adjacent to each other. Each group of permanent magnets is respectively secured to a different support plate having sound-passing perforations, and a support member secured to peripheral edges of the diaphragm to support it in tension is held between the peripheral portions of the support plates. In operation, an electric current signal at acoustic frequencies is made to flow through the coil on the diaphragm. As the coil is disposed in a magnetic field flowing along the plane of the diaphragm between adjacent permanent magnets, the signal causes the diaphragm to be subjected to an electromagnetic force in a direction perpendicular to the plane of the diaphragm to vibrate the diaphragm in response to the magnitude of the signal. The resulting audible sound wave passes through the perforations of the support plates.
In the electro-acoustic transducer according to the above-described U.S. patent, on the other hand, the support member for the diaphragm is to provide thereto with a surface rigidity by means of the tension but it is difficult to render the tension to be uniform over the entire diaphragm, the difficulty being increased as the vibratory area of the diaphragm expands. Accordingly, there have been involved such problems that the surface rigidity and tension of the diaphragm become likely to fluctuate and insufficient, resulting in the generation of abnormal sounds and unstable or unbalanced frequency characteristics, and the transducer is no more suitable for use in the acoustic apparatus and equipments.